Kim birdied two of her final three holes for a six-under 66 and a one-stroke lead from China's Yan Liu after Thursday's opening round of the HSBC Women's World Championship.
Thitikul won last week's tournament in her native Thailand and is among nine out of the top 10 players entered in the Singapore LPGA stop on the Sentosa Golf Club's Tanjong course.
But she battled to a 73, including five bogeys, to trail the leader by seven shots.
Hannah Green heads the seven-player Australian pack after she offset an eagle and three birdies with a double bogey and two bogeys in her round of 71 to be tied for 18th, with world No.4 Minjee Lee a shot behind.
Robyn Choi, Karis Davidson and Grace Kim all shot 73s with Steph Kyriacou and Cassie Porter posting 74s.
Five players were tied for third with 68s, including No.12-ranked Haeran Ryu.
Defending champion Lydia Ko had a 70.
No. 2-ranked Nelly Korda is the only player from the top 10 who is not in Singapore.
The tournament is the second of three stops on the LPGA's early-year Asian swing, with the final one next week at Hainan Island, China.
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